Reduce buffer size used while scanning media.

For some reasons, when playing a MOV file by AirPlay from an iPhone fails when the buffer is too big. So add a method for specific stream allowing to temporarily reduce the size of the buffer. I suspect an issue in FFmpeg caching code.
This allows to work around it, without investigating too much the "why".

Fixes #11263

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -982,6 +982,7 @@ int AvFormatDecoder::OpenFile(RingBuffer *rbuffer, bool novideo,
 
     InitByteContext();
 
+    ic->pb->buffer_size = ringBuffer->DiscoveryBufferSize();
     int err = avformat_open_input(&ic, filename, fmt, NULL);
     if (err < 0)
     {
@@ -990,6 +991,7 @@ int AvFormatDecoder::OpenFile(RingBuffer *rbuffer, bool novideo,
         return -1;
     }
 
+    ic->pb->buffer_size = ringBuffer->BestBufferSize();
     int ret = FindStreamInfo();
     if (ret < 0)
     {

mythtv/libs/libmythtv/ringbuffer.h

@@ -87,6 +87,7 @@ class MTV_PUBLIC RingBuffer : protected MThread
     virtual bool IsSeekingAllowed(void) { return true;  }
     virtual bool IsBookmarkAllowed(void) { return true; }
     virtual int  BestBufferSize(void)   { return 32768; }
+    virtual int  DiscoveryBufferSize(void)   { return BestBufferSize(); }
     static QString BitrateToString(uint64_t rate, bool hz = false);
     RingBufferType GetType() const { return type; }
 

mythtv/libs/libmythtv/streamingringbuffer.h

@@ -23,6 +23,7 @@ class StreamingRingBuffer : public RingBuffer
     virtual bool IsStreamed(void)       { return m_streamed;   }
     virtual bool IsSeekingAllowed(void) { return m_allowSeeks; }
     virtual bool IsBookmarkAllowed(void) { return false; }
+    virtual int  DiscoveryBufferSize(void)   { return 2048; }
 
   protected:
     virtual int safe_read(void *data, uint sz);